An ignition plug attached to an internal combustion engine utilizes non-equilibrium plasma (Japanese Unexamined Publication No. 2014-26754). A bottomed cylindrical insulator of the ignition plug is held by a metal shell and encloses the front end of a center electrode. When an electric discharge occurs between the metal shell and the center electrode of the ignition plug, a gas is ionized (non-equilibrium plasma) to cause an air-fuel mixture to generate a flame kernel. According to the conventional technology disclosed in Japanese Unexamined Publication No. 2014-26754, the outer diameter at the front side of the insulator is reduced to thin the wall thickness at the front side of the insulator in order to increase the amount of plasma generation per unit area at the front side, which is exposed in a combustion chamber, of the insulator.